In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
in prior art various materials have been proposed for use as OSCs in OFETs, including small molecules like for example pentacene, and polymers like for example polyhexylthiophene.
A promising class of conjugated small molecule semiconductors has been based upon the pentacene unit.[1] When deposited as a thin film by vacuum deposition, it was shown to have carrier mobilities in excess of 1 cm2 V−1 s−1 with very high current on/off ratios greater than 106.[2] However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films. Initial device fabrication was improved by adding solubilising groups, such as trialkylsilylethynyl, allowing mobilities >0.1 cm2V−1 s−1 [3]. It has also been reported that adding further substituents to the pentacene core unit can improve its semiconducting performance in field-effect transistor (FET) devices.[1]
However, the OSC materials of prior art, and devices comprising them, which have been investigated so far, do still have several drawbacks, and their properties, especially the solubility, processibility, charge-carrier mobility, on/off ratio and stability still leave room for further improvement.
Therefore, there is still a need for OSC materials that show good electronic properties, especially high charge carrier mobility, and good processibility, especially a high solubility in organic solvents. Moreover, for use in OFETs there is a need for OSC materials that allow improved charge injection into the semiconducting layer from the source-drain electrodes. For use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, and do especially show good processibility, good solubility in organic solvents and high charge carrier mobility. Another aim of the invention was to extend the pool of organic semiconducting materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
It was found that these aims can be achieved by providing compounds as claimed in the present invention. In particular, the inventors of this invention have found that compounds derived from anthra[2,3-b]benzo[d]thiophene,
which is disubstituted by ethynyl groups in 7- and 12-position, are suitable as semiconductors, exhibit very good solubility in most organic solvents, and show high performance when used as semiconducting layer in electronic devices like OFETs. It was found that OFET devices comprising such compounds as semiconductors show good mobility and on/off ratio values and can easily be prepared using solution deposition fabrication methods and printing techniques.
The asymmetrical anthra[2,3-b]benzo[d]thiophene unit has been previously prepared [4,5] and it was shown to have mobilities as high as 0.41 cm2V−1 s−1.[4] The high mobility was achieved by preparation of the device at room temperature allowing the possible use of plastic flexible substrates. Furthermore, according to single-crystal X-ray diffraction studies, the anthra[2,3-b]benzo[d]thiophene unit exhibits a herringbone arrangement [4], which is similar to that of pentacene[6].
However, the herringbone arrangement reported for the anthra[2,3-b]benzo[d]thiophene unit is not optimal for charge transport in FET devices. Another disadvantage of anthra[2,3-b]benzo[d]thiophene as reported in prior art is that the material is only moderately soluble in common organic solvents, which means that the compound is not ideal for solution processing by mass-production printing techniques such as ink-jet, gravure and flexo printing.
However, prior art does neither disclose nor suggest how anthra[2,3-b]benzo[d]thiophene could be modified to improve its properties in the way described above. In particular, prior art does not provide any hint that this could be solved by adding subtituents to the anthra[2,3-b]benzo[d]thiophene core, or to the type or exact position of possible substituents.